One of the key functions required for microsystems technologies used for biomedical applications is to separate specific cells or molecules from complex biological mixtures, such as blood, urine, or cerebrospinal fluid. For example, hemofiltration and hemadsorption techniques remove impurities or pathogens in blood. Various physical properties, including size (Huang et al., 2004; Yamada et al., 2004), motility (Cho et al., 2003), electric charge (Lu et al., 2004), electric dipole moment (Fiedler et al., 1998; Hunt et al., 2004), and optical qualities (Fu et al., 1999; Wang et al., 2005), have been studied to separate specific cells or molecules from these mixtures. Magnetic susceptibility also has been explored (Pamme, 2006) because magnetic sorting can be carried out at high-throughput in numerous biological fluids with minimal power requirements, and without damaging the sorted entities (Franzreb et al., 2006; Hirschbein et al., 1982; Lee et al., 2004; Safarik and Safarikova, 1999; Setchell, 1985). Biocompatible superparamagnetic particles are also now available with surfaces modified to promote binding to various molecules and cells. In fact, various macroscale magnetic sorting systems have been built and employed for research and clinical applications (Chalmers et al., 1998; Fuh and Chen, 1998; Handgretinger et al., 1998; Hartig et al., 1995; Melville et al., 1975a; Takayasu et al., 2000) (e.g., to isolate stem cells from batches of pooled blood for bone marrow reconstitution procedures in cancer patients (Handgretinger et al., 1998)).
Batch-type magnetic separators have been microfabricated on single chips that trap magnetic particles in flowing fluids using an external magnetic field, and then the particles are later eluted from the system (Ahn et al., 1996; Deng et al., 2002; Smistrup et al., 2005; Tibbe et al., 2002). However, the loading capacity of these devices is limited because accumulation of the collected particles can restrict fluid flow or lead to irreversible entrapment of samples, and the use of these systems is hampered by the need to disrupt continuous operation for sample elution.
Further, continuous on-chip separation may simplify microsystem operation and potentially improve separation efficiency. In particular, microfluidic systems that are extensively utilized in micro-total analysis systems (μTAS) offer the potential to separate components continuously from flowing liquids. Continuous separation of magnetic particles in microfluidic channels has been demonstrated by manually placing a permanent magnet or electromagnet beside a microchannel that contains multiple outlets (Blankenstein, 1997; Kim and Park, 2005; Pamme and Manz, 2004). However, because each magnet needs to be individually fabricated and positioned, further miniaturization and multiplexing is not possible with this approach.
Additionally, high-gradient magnetic concentrators (HGMCs) can generate a large magnetic force with simple device structures. Macroscale HGMCs have been used in magnetic separations for biomedical applications (Chalmers et al., 1998; Fuh and Chen, 1998; Hartig et al., 1995; Melville et al., 1975a; Takayasu, 2000), but are impractical for microsystems technologies due to their large dimensions. With the development of microfabrication technologies, it has become possible to microfabricate HGMCs along with microfluidic channels on a single chip. Several on-chip HGMC-microfluidic designs for continuous magnetic separation have been reported (Berger et al., 2001; Han and Frazier, 2004, 2006; Inglis et al., 2004). One design used microfabricated magnetic stripes aligned on the bottom of the fluid chamber to horizontally separate magnetically tagged leukocytes trapped on the magnetic stripes away from red blood cells (RBCs) flowing through the chamber (Inglis et al., 2004). In another design, a microfabricated magnetic wire was placed in the middle of the flow stream along the length of a single microfluidic channel, and used to separate deoxyhemoglobin RBCs from white blood cells based on the difference in their relative magnetic susceptibilities (Han and Frazier, 2006).
However, none of these designs provides for portable devices for in-field diagnosis or treatment of diseases caused by blood-borne pathogens, such as sepsis—the body's systemic response to infection in the blood. The overall death rate from this blood infection is 25% in the United States and higher internationally, and in the military field of operation. In a septic patient, the blood becomes overloaded with a rapidly growing infectious agent, and other clearance mechanisms are overcome. The prior attempts have fallen short in that no device presently exists that can rapidly clear infectious pathogens from blood and biological fluids without causing significant blood loss, obstructing blood flow, otherwise altering blood content, or compromising normal organ function.
Thus, there is a need for a device that can rapidly cleanse the blood of pathogens and other biological particulate materials without removing critical normal blood cells, proteins, fluids, or electrolytes. Also, there is a need for biocompatible magnetic labeling particles that will selectively bind to living pathogens within flowing blood such as natural opsonins, while being small enough so that they will not produce vascular occlusion. Moreover, there is a need for a microfluidic separator generating appropriate magnetic field gradients in a microfluidic device for continuous separation of materials from biological fluids in biomedical and biophysical diagnosis and treatment applications.